The arrest of four Muslim ex-convicts in an alleged homegrown terror plot in the Bronx is renewing fears about the spread of Islamic extremism in the nation's prisons.

At least two of the four men suspected of plotting to bomb synagogues and shoot down military airplanes converted to Islam behind bars. The alleged mastermind is also a convert, and the fourth man identified himself as a Muslim when he entered prison.

Islam has had a strong presence in U.S. prisons for decades, and many chaplains and corrections officials credit the faith, when taught properly, with being a stabilizing force that can help inmates turn their lives around.

But this week's foiled plot is not the first terror scheme implicating Muslim convicts, and it comes despite reports of progress in screening chaplains and materials on Islam in the prison system.

"Basically, the threat is real," said Paul Rogers, past president of the American Correctional Chaplains Association. "Prisons have unstable people and people who are on the edge of a lot of different things. The radical elements of any religion can be emphasized."